Thyroid hormones are critical for normal growth and development and for maintaining metabolic homeostasis (Paul M. Yen, Physiological reviews, Vol. 81(3): pp. 1097-1126 (2001)). Circulating levels of thyroid hormones are tightly regulated by feedback mechanisms in the hypothalamus/pituitary/thyroid (HPT) axis. Thyroid dysfunction leading to hypothyroidism or hyperthyroidism clearly demonstrates that thyroid hormones exert profound effects on cardiac function, body weight, metabolism, metabolic rate, body temperature, cholesterol, bone, muscle and behavior.
The biological activity of thyroid hormones is mediated by thyroid hormone receptors (TRs or THRs) (M. A. Lazar, Endocrine Reviews, Vol. 14: pp. 348-399 (1993)). TRs belong to the superfamily known as nuclear receptors. TRs form heterodimers with the retinoid receptor that act as ligand-inducible transcription factors. TRs have a ligand binding domain, a DNA binding domain, and an amino terminal domain, and regulate gene expression through interactions with DNA response elements and with various nuclear co-activators and co-repressors. The thyroid hormone receptors are derived from two separate genes, α and β. These distinct gene products produce multiple forms of their respective receptors through differential RNA processing. The major thyroid receptor isoforms are α1, α2, β1 and β2. Thyroid hormone receptors α1, β1 and β2 bind thyroid hormone. It has been shown that the thyroid hormone receptor subtypes can differ in their contribution to particular biological responses. Recent studies suggest that TRβ1 plays an important role in regulating TRH (thyrotropin releasing hormone) and on regulating thyroid hormone actions in the liver. TRβ2 plays an important role in the regulation of TSH (thyroid stimulating hormone) (Abel et. al., J. Clin. Invest., Vol 104: pp. 291-300 (1999)). TRβ1 plays an important role in regulating heart rate (B. Gloss et. al. Endocrinology, Vol. 142: pp. 544-550 (2001); C. Johansson et. al., Am. J. Physiol., Vol. 275: pp. R640-R646 (1998)).
Efforts have been made to synthesize thyroid hormone analogs which exhibit increased thyroid hormone receptor beta selectivity and/or tissue selective action. Such thyroid hormone mimetics may yield desirable reductions in body weight, lipids, cholesterol, and lipoproteins, with reduced impact on cardiovascular function or normal function of the hypothalamus/pituitary/thyroid axis (see, e.g., Joharapurkar et al., J. Med. Chem., 2012, 55 (12), pp 5649-5675). The development of thyroid hormone analogs which avoid the undesirable effects of hyperthyroidism and hypothyroidism while maintaining the beneficial effects of thyroid hormones would open new avenues of treatment for patients with metabolic disease such as obesity, hyperlipidemia, hypercholesterolemia, diabetes and other disorders and diseases such as liver steatosis and NASH, atherosclerosis, cardiovascular diseases, hypothyroidism, thyroid cancer, thyroid diseases, resistance to thyroid hormone and related disorders and diseases.
The present invention, in part, provides methods for synthesizing thyroid hormone analogs such as pyridazinone compounds and prodrugs thereof. An ideal method of synthesizing the thyroid hormone analogs and their prodrugs would, for example, provide product compounds in high purity and high yield. The present invention is directed at providing one or more of these desirable features.